Telecommunications channels which are constrained to operate with long pulse compression waveforms to reduce required peak power are generally limited in information rate. For example, when a waveform with constant amplitude A and length T is employed, phase modulation and M-ary encoding can be used to transfer digital code words and recover information, but at a rate which is slowed as waveforms are lengthened.
Several prior art methods of transmitting signals using pulse code modulation exist. FIG. 1 shows a generic version of one type of conventional system. A transmitter 10, which encodes a digital input word onto a carrier signal by phase modulation, includes a waveform generator 12 responsive to the digital input word for selecting from a number of different waveforms. Waveform generator 12 outputs analog waveforms sj(t), which are modulated onto a radio frequency carrier in the form of a long pulse compression waveform by a modulator 14. Modulated waveforms from modulator 14 are amplified by an amplifier 15 and transmitted over a transmission link using an antenna 16.
The transmitted long pulse compression waveform is received by a receiver 20, which includes a low noise amplifier 21 and a converter 22 responsive to the output of amplifier 21. Converter 22 demodulates the received waveform back into an analog signal sj(t) with a noise component n(t). A coherent analog-to-digital converter (ADC) 23 converts the analog waveform sj(t)+n(t) into a digital signal. The first transmitted waveform s.sub.o (t) is a timing pulse, which is processed by matched filter ho(t) in bank 24 and output to constant false alarm rate (CFAR) and timing circuitry 25. Circuitry 25 identifies the timing pulse S.sub.0 (t) and generates a timing signal T . The bank of matched filters 24 decodes the digital signal by computing the correlation between the received digital signal and M codes stored in filter bank 24. A decoder circuit 26, in response to signal T.sub.s and the correlation values generated by filter bank 24, selects the largest correlation value and outputs a code corresponding to the code producing that correlation value.
Examples of the generic system shown in FIG. 1 are disclosed in, e.g., U.S. Pat. No. 4,774,715, which discloses a system using a time shifted pseudo-noise code and an amplitude comparator to decode an incoming signal; U.S. Pat. No. 4,550,414, which discloses a method of automatically detecting a correlation signal using a matched filter arrangement; and U.S. Pat. Nos. 4,225,935 and 4,866,734, which disclose similar receiver methods using correlators and convolvers.
M-ary encoding and phase modulation, where M=256, allows encoding information within a signal, which is based on long pulse compression waveforms, to thereby recover a significant portion of the information rate transmission potential lost by using phase modulation encoding. Heretofore a high data rate communications system using long pulses and 256-ary coding has not been produced.